FR2886640A1 - METHOD AND PREFORM FOR THE PRODUCTION OF COMPOSITE MATERIAL PARTS BY CVI DENSIFICATION AND PIECES OBTAINED - Google Patents

Abstract

A piece of composite material is produced by forming a fibrous preform (20), forming holes (22) extending into the preform from at least one face thereof, and densifying the preform by a matrix formed at least partially by chemical vapor infiltration (CVI). The holes (22) are formed by removal of the material therefrom with fracture of the fibers, for example by pressurized water jet machining, the arrangement of the fibers in the preform having holes being substantially unmodified with respect to to the initial arrangement before forming the holes. The CVI densification gradient is thus very small.

Description

Background of the invention

  The present invention relates to the production of composite material parts comprising the formation of a fibrous substrate and the densification thereof by a matrix formed by chemical vapor infiltration, or CVI ("Chemical Vapor Infiltration"). A particular, but not exclusive, field of application of the invention is the production of brake discs made of carbon / carbon (C / C) composite material, in particular for aeronautical brakes. The invention is however applicable to the production of other parts in composite material C / C or other composite material, in particular ceramic matrix composite material (CMC).

  CVI densification of porous substrates, such as fibrous substrates or preforms, is well known. The substrates to be densified are placed in an oven. A reaction gas phase is admitted into the furnace to deposit the constituent material of the matrix within the porosity of the substrates by decomposition of one or more components of the gas phase, or reaction between several constituents, under conditions of temperature and humidity. determined pressure.

  A major difficulty of CVI processes is to minimize the densification gradient within the substrates to obtain parts with the least possible inhomogeneity of properties in their volume.

  Indeed, the deposition of the matrix tends to form preferentially in the surface portions of the substrates encountered first by the reaction gas phase. This results in a depletion of the gas phase which manages to diffuse into the core of the substrates and a premature closure of the porosity of the surface portions of the substrate which progressively reduces the diffusion possibilities in the core of the gas phase. A densification gradient is therefore established between the surface portions and the core of the substrates.

  Therefore, especially for the production of thick parts, it is in practice necessary, after having reached a certain stage of densification, to interrupt the process to remove the partially densified substrates in order to perform surface machining, or peeling, allowing to re-open the superficial porosity. Densification can then be continued, with easier access to the core of the substrates for the diffusion of the reaction gas phase. In the case for example of the production of brake disks, it is generally carried out for example two cycles of densification CVI (cycles I1 and I2) separated by peeling. In practice, however, a densification gradient is observed on the pieces finally obtained.

  It is certainly known, to avoid the generation of a densification gradient and possibly avoid scaling operations, to implement a temperature gradient CVI densification process, that is to say by heating the substrates of non-uniform way. Non-uniform heating by direct coupling between an inductor and one or more annular substrates to be densified is described in documents US Pat. Nos. 5,846,611 and EP 0,946,461. Deposition of the matrix in zones of the substrates least easily accessible by the phase gas is favored by bringing these areas to a higher temperature than other parts of the substrates. However, this technique is limited to certain forms and types of substrates and to certain arrangements of substrates loadings in the furnace.

  It has been proposed in US Pat. No. 5,405,560 to promote the access of the reaction gas phase within annular fibrous preforms for C / C composite brake discs by providing holes-shaped passages extending through the preforms, between their opposite faces. These holes are formed by the introduction of needles that repel the fibers of the preforms without damaging them. During CVI densification, the holes provide the gas phase with a shortened path to reach the central parts of the preforms. Tests carried out by the Applicant, however, have shown the limits of this technique in minimizing the densification gradient, as described below.

  The formation of holes in brake disk blanks made of C / C composite material is also described in document FR 2 144 329.

  However, this document relates to the densification of fibrous preforms of liquid brake discs, that is to say by impregnating the preforms with a carbon precursor resin, which resin is crosslinked (cured) and then carbonized or graphitized to form the carbon matrix. Holes are formed after curing the resin and before carbonization or graphitization to promote the evacuation of volatile species during carbonization or graphitization and thus avoid that gas is trapped in the carbon matrix. This is a totally different process from CVI densification.

  OBJECT OF THE INVENTION The object of the invention is to provide a CVI densification process that allows for an almost uniform densification of fibrous substrates for the manufacture of composite material parts.

  This object is achieved by a method comprising forming a fibrous preform, forming holes extending into the preform from at least one face thereof, and densifying the preform with a matrix formed at at least partially by chemical vapor infiltration, in which, in accordance with the invention, the holes are formed in the preform by removal of the material therefrom with breaking of the fibers, the arrangement of the fibers in the preform provided with holes being substantially unmodified with respect to the initial arrangement before hole formation.

  As will be shown later, the formation of holes in the substrate by removal of material surprisingly makes it possible to obtain an almost uniform densification of the substrate, whereas such a result is far from being obtained when the holes are formed by insertion of needles having a non-destructive effect on the fibers, as in the prior art.

  The holes can be formed by mechanical machining with water jet under high pressure.

  According to another embodiment of the method, the holes may be formed by localized thermal action having a destructive effect on the fiber material, possibly in connection with exposure to an oxidizing medium. This may be the case in particular for carbon fibers. The localized thermal action can be produced by laser radiation.

  The holes may pass through the substrate, between two faces thereof or be blind holes opening on only one side of the substrate.

  Moreover, the holes may be orthogonal to a substrate surface on which they open, or in a non-orthogonal direction.

  The average diameter of the holes is chosen to prevent them being closed by the deposition of the matrix material before the end of the CVI densification process. An average diameter of between about 0.05 mm and 2 mm may for example be chosen.

  The density of the holes is chosen to be sufficient to provide a short path to the reaction gas phase to all parts of the substrate that it is desired to densify in a substantially uniform manner. A density of between about 0.25 holes / cm 2 and 4 holes / cm 2 can for example be chosen, this density being measured in number of holes per unit area in a median plane of the substrate. Expressed otherwise, the distance, or not, between axes of neighboring holes is preferably between 0.5 cm and 2 cm.

  The invention also relates to a fibrous preform for producing a composite material part, comprising holes which extend into the preform from at least one face thereof, a preform in which the density of the fibers in the vicinity of the hole walls of the preform is not substantially greater than the density of the fibers in other parts of the preform.

  According to a feature of the preform, the holes are delimited by boundary zones of rupture or elimination of fibers.

  The invention also relates to a composite material part comprising a fibrous reinforcement densified by a matrix obtained at least partially by chemical vapor infiltration and having holes which extend into the part from at least one side thereof. ci, part in which, according to the invention, the volumetric density of the reinforcing fibers in the vicinity of the walls of the holes is not substantially greater than the density of the fibers in other parts of the room.

Brief description of the drawings

  The invention will be better understood on reading the description given below, by way of indication but not limitation, with reference to the accompanying drawings, in which: - Figure 1 shows the successive steps of producing a composite material part according to a mode of implementation of a method according to the invention; - Figure 2 shows schematically, in perspective, an annular fiber preform brake disc in which holes are formed; - Figure 3 is a partial sectional view on an enlarged scale along the plane III of Figure 2; Figures 4 to 6 are sectional views showing alternative hole formation in an annular fibrous disk brake preform; - Figure 7 shows schematically a brake disc obtained after densification, CVI and final machining, with a preform such as that of Figure 2; Figures 8 to 10 show alternative arrangements of holes on the surface of a fibrous preform; and FIG. 11 very diagrammatically shows a stack loading of annular fibrous preforms of brake disks in a CVI densification furnace.

  DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION A first step 10 of the method shown in FIG. 1 consists in producing a three-dimensional fibrous substrate (3D), or fibrous preform, having a shape close to that of a composite material part. get. The techniques for producing such fibrous preforms are well known.

  It is possible to start from unidimensional fibrous elements (IDs) such as wires or cables that are wound on a form or a mandrel or that are used to form a 3D substrate directly by three-dimensional weaving, knitting or braiding.

  It is also possible to start from two-dimensional (2D) fibrous textures such as fabrics, knits, flat braids, thin felts, unidirectional sheets (UD) formed of yarns or cables parallel to each other, or multidirectional sheets (nD) formed of UD sheets. superimposed in different directions and interconnected for example by light needling or sewing. Strata formed of such 2D textures are superimposed by winding on a form or mandrel or by draping on a shape or a support, and are interlinked for example by needling, sewing or by thread implantation through the strata, for get a 3D substrate.

  A 3D substrate can also be obtained in the form of thick felt made by needling staple fibers randomly oriented.

  A 3D substrate thus obtained can be used directly as a fibrous preform of a part to be obtained. It is also possible to form a desired fibrous preform by cutting a 3D substrate to obtain the desired shape.

  The constituent fibers of the preform are chosen according to the application of the composite material part to be produced. In the case of thermostructural composite materials, that is to say materials having good mechanical properties and ability to retain them at high temperatures, the fibers of the fibrous reinforcement of the material are typically carbon or ceramic. The preform can be made from such fibers, or from carbon precursor fibers or ceramic which may be more suitable to support various textile operations for the realization of 3D fibrous substrates. In the latter case, the transformation of the precursor into carbon or ceramic is performed after formation of the substrate or preform, usually by heat treatment.

  A second step 12 of the method consists in forming in the preform holes for facilitating access of a reaction gas phase to the heart of the preform during a subsequent CVI densification. When the preform is made of fibers made of a material obtained by transformation of a precursor material, the holes may be formed in the preform after transformation of the precursor or before this transformation. In the latter case, account will be taken of any dimensional shrinkage during the conversion of the precursor to obtain desired hole dimensions.

  Figures 2 and 3 show an annular fibrous preform 20 made of carbon fibers for manufacturing a brake disc of composite material C / C. Such a preform can be obtained by cutting a 3D plate-shaped fibrous substrate made for example by superposition and needling of fabric layers or unidirectional or multidirectional sheets of preoxidized polyacrylonitrile (PAN) fibers, precursor of carbon. The preform can also be obtained by superimposition and needling of annular layers cut in fabrics or unidirectional or multidirectional sheets of preoxidized PAN fibers. After formation of the annular preform in preoxidized PAN fibers, the conversion of the preoxidized PAN to carbon is carried out by heat treatment. For example, documents US Pat. Nos. 4,790,052 and 5,792,715 may be referred to.

  Holes 22 are formed in the preform 20 parallel to its axis 21 and extend over its entire thickness between the opposite faces 20a and 20b on which they open.

  Alternatively, as shown in Figure 4, blind holes 22a, 22b are formed in the preform, the holes 22a opening only on the face 20a while the holes 22b only open on the face 20b. However, it is noted that the holes 22a, 22b extend over a very large part of the thickness of the preform.

  According to another variant, the holes may be formed at an angle, that is to say with their axes forming a non-zero angle with respect to the normal to the faces 20a, 20b or the axis of the preform 20, that it 22c (FIG. 5) or blind holes 22d, 22e (FIG. 6).

  According to a characteristic of the process according to the invention, the holes are formed in the preform by removal of material.

  For this purpose, a pressurized water jet drilling technique can be used which can be used to form through holes or blind holes. The water used may or may not be charged with solid particles. The drilling can be carried out in one or more pulses of water jet, or continuously. When the diameter of the holes is relatively large compared to that of the jet, the drilling of a hole can be achieved by trimming, that is to say by successive cuts along the circumference of the holes to be made. According to the implementation of the drilling technique, the holes may have a slightly frustoconical shape as shown in FIGS. 4 and 6. The diameter of the holes is then increasing from the face on the side of which the jet machining water is practiced due to the dispersion of the water jet. In the case of through holes, approximately 50% of the holes will then be machined from one face and the other holes from the other face so as to have a void density created by the holes substantially uniform throughout the hole. thickness of the preform. For the same purpose, in the case of Figure 4, about the same number of holes are formed from each face of the preform.

  Another possible hole-forming technique, suitable in the case where the fiber material is heat-removable, is to produce a localized thermal action, for example by laser radiation. With in particular carbon fibers, this thermal action in an oxidizing medium, for example in air, allows oxidation removal of the fiber material.

  The formation of the holes by removal of material has a destructive effect on the fibers of the preform but without altering the arrangement of the fibers in the vicinity of the walls of the holes relative to the initial arrangement before formation of the holes. Thus, the holes formed are delimited by zones of rupture or elimination of the fibers and the density of the fibers of the preform in the vicinity of the walls of the holes is not increased, contrary to what would be the case if the holes were formed by inserting fibers-repelling needles into the wall areas of the holes.

  In the subsequent CVI densification process, the access of the reaction gas phase to the core of the fibrous preform material is not restricted through the walls of the holes, as compared with the external surfaces of the preform, as would be the case. case if the fibers were pushed back into the wall areas for hole formation.

  Premature closure of the walls of the holes, which would deprive them of their effectiveness, is thus avoided during the course of the densification process.

  In the preform, as well as in the composite material part obtained after densification CVI, the fiber density density, in the vicinity of the walls of the holes, is not substantially greater than the density of fibers in other parts of the preform and the room. This does not create an inhomogeneity in the properties of the composite material.

  The average diameter of the holes is chosen large enough to prevent them from being closed before the end of the CVI densification and then no longer fulfill their function, but while remaining limited so as not to affect the behavior of the composite material part obtained after densification.

  This average diameter may therefore vary depending on the thickness of the matrix to be deposited on the fibers, the dimensions of the part to be produced and the use thereof.

  In general, and in particular for aeronautical brake disc preforms, the average diameter of the holes may be chosen equal to a value of between about 0.05 mm and 2 mm.

  The density of the holes is chosen sufficient, in connection with their diameter, to offer a short path to the reaction gas phase to any part of the preform during densification CVI, but while limited to not affect the behavior of the composite material part obtained after densification. As for the diameter, this density may vary depending on the dimensions of the part to be made and its use.

  In general, and in particular for aeronautical brake disc preforms, the density of the holes may be chosen equal to a value of between about 0.25 holes / cm 2 and 4 holes / cm 2. This density is measured in a median plane of the preform to cover the case where blind holes are formed. It can also be measured on one side in the case of through holes. In another way, it is preferable for the distance, or not, between axes of neighboring holes, a value between 0.5 cm and 2 cm.

  The diameter of the holes in a preform may be constant or not. It is the same for the density of the holes.

  After formation of the holes, the densification of the preform is carried out by CVI (step 14). CVI densification processes by carbon or ceramic matrices are well known. A reaction gas phase adapted to the nature of the matrix material to be deposited is used.

  Depending on the case, in particular depending on the thickness of the preform to be densified, peeling at least the exposed faces of the preform may not be desirable. If such peeling is performed, step 14 comprises a first densification cycle I1 followed by surface machining of the preform and a second densification cycle I2.

  FIG. 7 shows a brake disk 26 as obtained after densification CVI of a preform such as that of FIG. 2 and final-sized machining with notch formation 26c allowing the mechanical connection of the disk with an axis. In this example, it is a rotor disc having two opposite friction faces 26a, 26b. It will be noted that holes 28 corresponding to the holes formed in the preform are visible.

  In the example illustrated in Figures 2 and 7, holes are formed throughout the volume. It will be possible, in a variant, to limit the formation of the holes in certain zones of the preform, for example, in the case of a brake disc, to the zones corresponding to the friction faces.

  Although the case of annular fibrous preforms for brake disks has been envisaged, it is clear that the invention is applicable to all types of preforms intended for the production of composite material parts, in particular of thick parts for which the problem of uniform densification arises.

  In Figure 2, the holes are arranged at regular intervals along concentric circles. They could be arranged along a spiral line. In addition, whether in the case of annular fibrous preforms or other forms of fibrous preforms, the holes may be arranged according to still other reasons, for example at quadrangles (FIG. 8), vertices and centers. quadrangles (figure 9), at the vertices of hexagons (figure 10), ...

  In addition, the invention is applicable regardless of the nature of the fibers of the preforms and the matrix deposited to densify them by CVI.

  It will also be noted that the densification of the preform may comprise a first phase of partial densification by a liquid route before a second CVI densification phase. Liquid densification, as is well known, consists in carrying out at least one impregnation cycle of the preform with a liquid composition containing a liquid precursor of the matrix material. The precursor is typically a resin, for example an organic precursor carbon resin. After drying, for removal of a possible solvent, and polymerization of the resin, a precursor transformation heat treatment is carried out.

Example

  Carbon fiber fibrous annular preforms for aeronautical brake discs were made as follows.

  Multidirectional layers were obtained by superposition of three unidirectional sheets of preoxidized PAN fibers forming between them angles of 60 and linked together by needling. The multidirectional plies were superimposed and needled as they were superimposed to obtain a needled plate in which annular preforms in preoxidized PAN were cut.

  The preoxidized PAN preforms were subjected to a heat treatment at about 1600 ° C to convert PAN to carbon. Carbon fiber annular preforms having inner and outer diameters of 26 cm and 48 cm, a thickness of 3.5 cm and a fiber volume content of about 23% were then obtained, the fiber volume ratio being the percentage occupied by the fibers of the apparent volume of the preform.

  Some preforms have been pierced with through holes formed by pressurized water jet, with a density of about 1 hole / cm 2. Preforms were thus obtained with holes of respective diameters of about 0.2 mm for preforms A1, A2, of about 0.5 mm for preforms B1, B2 and about 1 mm for preforms C1, C2.

  For comparison, holes were made in another preform D by inserting needles with a diameter of 2 mm, with a density of 1 hole / cm 2, the needles then being removed for CVI densification.

  An annular stack-shaped preform charge consisting essentially of non-drilled preforms E has been formed by inserting preforms A1, A2, B1, B2, C1, C2 into the stack between two non-drilled preforms E1 and E2.

  FIG. 11 shows such a stack feed 30 introduced into the reaction chamber 32 of a CVI densification furnace to effect CVI densification of the "directed flow" type as described in US 5,904,957. Briefly, the furnace is heated by inductive coupling between an inductor 34 and a graphite susceptor 36 delimiting the reaction chamber. A reaction gas phase is admitted through the bottom of the susceptor 36, passes through a preheating zone 37 and is directed into the internal volume 31 of the closed stack at its upper part. The gaseous phase flows to the internal volume of the chamber 32 outside the cell 30 by passing through gaps provided by spacers between the preforms, and diffusing therethrough. The effluent gas is extracted through the susceptor lid by suction by means of a pumping unit which sets the desired level of pressure in the chamber.

  The CVI densification of the substrates by a pyrolytic carbon matrix was carried out using a reaction gas phase based on natural gas, at a pressure of about 5 kPa and at a temperature of about 1000 C.

  The densification was carried out in two cycles I1, I2 separated by a peeling operation for which the load was removed from the oven. The cycle I1 was carried out under predetermined conditions making it possible to increase the density of the preforms E to a value of approximately 1, 6. After machining the principal faces of the partially densified preforms to bring them to a thickness close to that of the discs to achieve, the cycle I2 was performed under predetermined conditions to bring the density to about 1.8. For the cycle I2, the loading of the furnace was carried out by placing again the preforms E1, A1, A2, B1, B2, C1, C2 and E2 partially densified in this order.

  A two-cycle densification I1 and I2 of a stacked charge formed of E-type substrates with the exception of a substrate D inserted in the stack adjacent to a substrate E3 was similarly carried out.

  Table I below shows the density values measured for the Al, A2, B1, B2, C1, C2, E1 and E2 disks after the cycles I1 and I2 and for the disks E3 and D after the cycle I2. It can be seen that the final densities obtained for the discs A1, A2, B1, B2, C1 and C2 are substantially greater than those of the discs D, E1 and E2, and that the density of the discs D is far from being increased in the same proportions at the end of cycle 12, with respect to the density of the disk E3.

Table I

  Preform Holes Density at the end Density at the end of cycle start II Cycle I2 El None 1.58 1.81 Al 0 0.2 mm 1.59 1.88 A2 0 0.2 mm 1.56 1, 88 B1 0 0.5 mm 1.56 1.89 B2 0 0.5 mm 1.57 1.89 Cl 0 0.1 mm 1.57 1.89 C2 0 1 mm 1.59 1.89 E2 Nil 1.61 1 , 80 E3 None 1.79 D Insertion 1.81 needles 0 2 mm In order to verify the existence or not of a densification gradient, the Al, El, D and E2 disks obtained after cycle I2 have been cut off, blocks of substantially parallelepiped shape along a radius of the disks. For each block, the density was measured in different zones Z1 to Z5 between the inside diameter and the outside diameter, in the vicinity of one face, in the vicinity of another face, and in the medial radial portion.

  Table II below shows the density values recorded. The remarkable result obtained with the Al disk produced according to the invention is noted since the density is practically uniform (variation of less than 1.7%).

  With the El and E3 disks obtained from a preform without holes, a significant variation in density is observed, reflecting the existence of a fairly strong densification gradient despite intermediate peeling (variation of 8.1% and 7.70 / 0 respectively).

  A density variation of 6% is measured on the disk D, which is smaller than that observed on the El and E3 disks, but nevertheless still very substantial.

Table II

  Preform Holes Density at end of start cycle I2 Z1 Z2 Z3 Z4 Z5 (outside radius) face 1.86 1.87 1.88 1.87 1.87 Al center 0 0.2 mm 1.86 1, 86 1, 86 1.85 1.85 face 1.87 1.87 1.87 1.86 1.86 face 1.83 1.77 1.78 1.79 1.84 El center none 1.80 1.69 1, 70 1,69 1,78 face 1,81 1,76 1,76 1,78 1,82 face Insertion 1,82 1,79 1,77 1,79 1,80 D center needles 1,77 1,72 1 , 71 1.72 1.79 face 0 2 mm 1.79 1.77 1.76 1.78 1.79 face 1.80 1.78 1.75 1, 77 1.81 E3 center none 1.75 1 , 70 1.67 1.71 1.78 face 1.75 1.73 1.74 1.76 1.81 Thus the process according to the invention is remarkable in that it makes it possible to increase the degree of densification ( therefore, for the same density objective, to reduce the densification time), while eliminating almost the densification gradient, results that the method of the prior art (formation of holes by insertion of needles) does not allow get.

Claims (13)

  A method of making composite material parts comprising forming a fibrous preform, forming holes extending into the preform from at least one face thereof, and densifying the preform with a matrix at least partly formed by chemical vapor infiltration, characterized in that the holes are formed in the preform by removal of the material therefrom with breaking of the fibers, the arrangement of the fibers in the preform with holes being substantially modified from the initial arrangement before hole formation.   2. Method according to claim 1, characterized in that the holes are formed by machining with water jet under pressure.   3. Method according to claim 1, characterized in that the holes are formed by localized thermal action on the fiber material of the preform.   4. Method according to claim 3, characterized in that the holes are formed under the effect of laser radiation.   5. Method according to any one of claims 3 and 4, characterized in that the holes are formed by removal of the fiber material by oxidation.   6. Method according to any one of claims 1 to 5, characterized in that the holes have an average diameter of between 0.05 mm and 2 mm.   7. Method according to any one of claims 1 to 6, characterized in that the density of the holes in a median plane of the substrate is between 0.25 holes / cm2 and 4 holes / cm2.   8. Method according to any one of claims 1 to 6, characterized in that the distance between axes of adjacent holes is between 0.5 cm and 2 cm.   Fibrous preform (20) for producing a composite material part, having holes (22; 22a, 22b) which extend into the preform from at least one face thereof, characterized in that the density of the fibers in the vicinity of the hole walls of the preform is not substantially greater than the density of the fibers in other parts of the preform.   10. Preform according to claim 9, characterized in that the holes are delimited by boundary areas of rupture or removal of fibers.   11. Preform according to any one of claims 9 and 10, characterized in that the holes have a mean diameter of between 0.05 mm and 2 mm.   12. Preform according to any one of claims 9 to 11, characterized in that the density of the holes in a median plane of the substrate is between 0.25 holes / cm2 and 4 holes / cm2.   13. Composite material part (26) comprising a matrix-densified fibrous reinforcement obtained at least partially by chemical vapor infiltration and having holes (28) extending into the part from at least one face of this, characterized in that the volumetric density of the reinforcing fibers in the vicinity of the walls of the holes is not substantially greater than the density of the fibers in other parts of the room.